Follow-up from "Change increase_usage to an instance method"

The following discussion from !85939 (merged) should be addressed:

• @drew started a discussion: (+2 comments)

  This should probably go in a follow-up, for the sake of keeping the refactors ultra-clear that there's no behavioral change, but being able to consolidate this validates the class/object API change to me.

        def track_monthly_usage(consumption, duration)
          # preload minutes usage data outside of transaction
          usages = [project_usage, namespace_usage].compact
  
          ::Ci::Minutes::NamespaceMonthlyUsage.transaction do
            usages.each { |usage| usage.increase_usage(amount_used: consumption, shared_runners_duration: duration) }
          end
        end